The present invention relates to a low fuel indicating device for an airplane or the like and, more particularly, to such a device which will provide a temporary supply of fuel after low fuel is indicated to allow sufficient time for corrective action.
The need for such a device is demonstrated in the Federal Aviation Administration (FAA) reports on aviation accidents. There are a number of accidents which occur because of fuel exhaustion, and yet FAA accident investigations find ample fuel remaining aboard the aircraft. The pilot either forgot to switch to the fullest tank or switched to an empty tank. Many of these fuel starvation accidents occur at low altitudes, e.g., during landing and takeoff flight configurations where there is not sufficient time to restart an engine. It usually takes time to restart an engine and, without power at low altitude, the aircraft is apt to make contact with the ground before engine power can be restored.
Loss of power because of fuel exhaustion at reasonably high altitudes is not apt to result in an accident. Even though altitude is lost, time is afforded in which to switch to an alternate fuel source or respond by implementing emergency procedures. However, some engines, owing to various reasons, are difficult or nearly impossible to restart while in flight, and an emergency landing is the only recourse which can be executed successfully if conditions permit proper execution of these procedures.
In a typical aircraft fuel system, a pilot has manual duties to perform to manage fuel consumption and ensure proper engine operation. By the use of a fuel selector valve for multiple fuel tanks and an electric fuel pump, for example, the pilot can maintain a fuel flow to the engine to ensure power. The objective of operating these devices is to manage fuel so that at the end of the flight one tank contains the majority of the fuel remaining on board. Landing, while operating from the fullest tank, ensures an adequate fuel supply to an engine. Landing maneuvers may uncover the outlet of a partially filled tank and allow the fuel system to take in vapor which can cause a temporary or complete power failure.
In many aircraft fuel systems the electric fuel pump is a back-up to the normal engine fuel pump in case it fails. The electric fuel pump is used to insure fuel flow during takeoffs and landings, and is also turned on during normal flight conditions prior to operating the fuel selector valve to insure that there is no momentary interruption in fuel flow.
Fuel quantity gauges are used to show how much fuel remains aboard the aircraft. In most cases, these gauges are similar to the types used in automobiles and other vehicles. At best they are an indication with questionable accuracy, especially when the needle approaches the empty mark, the problem being that the fuel tank may run dry prior to the gauge indicating empty. Such gauges may indicate properly or may indicate empty with a large quantity of fuel remaining in the tank. These inaccuracies are not serious in automobile operation; however, they present a serious problem in aircraft operation, especially near the end of a flight when fuel quantities can be low.
Aircraft flight is not always a smooth ride. As much as a passenger may be bounced about during a rough ride, so is the fuel because neither is rigidly secured to the air-frame. It can be envisioned, under these circumstances, that certain amounts of fuel and vapors in the fuel tanks become mixed (vapor entrainment). A full tank will be subject to less vapor entrainment than a partly empty tank because of the availability of vapor volumes. Therefore, it is common for entrained vapors to be ingested by the engine and, by one means or another, engines are designed to compensate for this phenomenon. It will be understood that space above liquid surface in a fuel tank is referred to herein as vapor, a mixture of atmospheric air and fuel fumes--a combustible mixture of these two gases. Certain amounts of these vapors are either burned in the engine or vented. However, a large amount of vapor, substituted as fuel, will cause the engine to lose power to stop operating.
Although the basic aircraft fuel system has been dealt with as a single engine arrangement, the same explanations and problems apply to multi-engine aircraft. Each engine of an aircraft may have a fuel system similar to that described herein, the exception being that some high-wing aircraft, with fuel tanks in the wings, may eliminate the electric fuel pump because of the availability of gravity feed. In the latter case, however, the flight operations are faced with the possible fuel starvation problems previously explained.